FLAC is an Open Source lossless audio codec developed by Josh Coalson from 2001 to 2009. From 2012 to 2021 it was maintained by Erik de Castro Lopo. It continues to be maintained by various volunteers under the auspices of the Xiph.org Foundation.
You can git clone this project into the Arduino libraries folder with
cd ~/Documents/Arduino/libraries
git clone https://github.com/pschatzmann/arduino-libflac.git
I recommend to use this library together with my Arduino Audio Tools. This is just one of many codecs that I have collected so far: Further details can be found in the Encoding and Decoding Wiki of the Audio Tools.
This library requires arduino-libopus